1. Field of the Invention
The present invention relates to a method and a system for enhanced ratio control in a continuously variable transmission.
2. Description of the Background Art
Continuously variable transmissions (CVT's) are transmissions that change a speed ratio continuously, not in discrete intervals. This continuous nature of CVT's gives them an infinite number of speed ratios, making them very attractive for automotive use.
Various types of CVT are known. One such example is a CVT with pulley/V-belt power transfer. Another example is a CVT with disc/roller power transfer. The CVT of this type is often referred to as a toroidal-type CVT (TCVT) because it transmits torque from one rotating semi-toroidal disc to another semi-toroidal disc by traction rollers through a traction force. The two semi-toroidal discs form a toroidal cavity. In each toroidal cavity, it is preferred to have two traction rollers in equiangularly spaced relationship engaging the discs for transmission of motion therebetween. While three or four traction rollers may be disposed in spaced relationship in each toroidal cavity and will provide increased life for contact surfaces as the total surface area is increased, two traction rollers are preferred for simplicity.
Each traction roller is rotatably supported by a pivot trunnion, respectively. The pivot trunnions, in turn, are supported to pivot about their respective pivot axis. In order to controllably pivot the pivot trunnions for a ratio change, a hydraulic control means is provided. The hydraulic control means is included in a hydraulic cylinder at each pivot trunnion and includes a control volume defined in the hydraulic cylinder between a piston and an axial end of the hydraulic cylinder. The pistons within the hydraulic cylinders are connected to the pivot trunnions along their pivot axis by rods. The piston and its associated rod are thereby rotatable about the pivot axis with the associated pivot trunnion. Variation of the control volume causes the piston to move relative to the hydraulic cylinder, and applies a control force to displace the pivot trunnions. Control forces applied displace the pivot trunnions in the opposite directions along their pivot axis. As a result, the pivot trunnions are caused to pivot about their respective pivot axis, due to the forces present in the rotating toroidal discs, for initiating ratio change.
For terminating the ratio change when a desired ratio has been obtained, a feedback structure is provided. The feedback structure preferably includes a source of hydraulic pressure, and a ratio control valve for controlling the flow of hydraulic fluid for initiating ratio change. The feedback structure further includes a mechanism associated with at least one pivot trunnion to adjust the ratio control valve upon pivotal movement of the pivot trunnion to a desired ratio. The mechanism is preferably a cam connected to a pivot trunnion. The cam may be linked mechanically and/or electronically to operate the ratio control valve upon reaching a desired rotation.
In CVTs, a measured value of CVT involves noise, which may cause hunting in CVT ratio. Suppression of such hunting is proposed by JP-A 8-277927.
According to this known technique, a dead zone is provided. The width of dead zone is variable in response to revolution speed of CVT input or output member or the product of revolution speeds of CVT input and output members. The dead zone is compared to a deviation between a desired value of CVT ratio and a measured value of CVT ratio. When the deviation falls in the dead zone, a ratio change is suspended. The measured value of CVT ratio inevitably involves noise because trains of output pulses of revolution speed sensors are used in calculating a ratio between a revolution speed of a CVT input member and a revolution speed of a CVT output member.
This section is a description on noise inevitably included in a measured value of CVT ratio. Each unit of CVT uses revolution speed sensors, each of which includes a toothed wheel coupled to an input or output shaft, and a Hall element sensor. The sensors generate trains of pulses. A filter is provided to deal with noise contained in the measured value of CVT ratio. Due to product-by-product variability in processing accuracy of toothed wheels, the width of noise after filtering may differ from one unit to another. This noise problem has not been solved by the prior art because the width of a dead zone is controlled in an open loop manner. The width of the dead zone is difficult to nicely fit all noise situations derived from difference in the width of noise from one unit to another. To clearly illustrate this difficulty, FIGS. 26A, 26B and 26C have been prepared to consider, two cases, namely case 1 and case 2. FIG. 26A shows typical pattern of noise after filtering and dead zones of cases 1 and 2. As shown in FIG. 26A, the width of dead zone of case 1 is narrower than the width of dead zone of case 2. FIG. 26B illustrates a problem inherent with the case 1, while FIG. 26C illustrates a problem inherent with the case 2. As shown in FIG. 26B, the dead zone is exceeded, causing hunting of actuator command that leads to undesired pulsating ratio change. As shown in FIG. 26C, there remains a standing deviation from a desired value of CVT ratio.
A need remains for improving the conventional CVT ratio control to remove at least one of the following insufficiencies.
a) Due to hunting or standing wave, a CVT cannot always establish, with good precision, a predetermined value of CVT ratio required for moving the vehicle from standstill. In this case, the driver feels unpleasantly a difference in vehicle acceleration upon starting the vehicle. This predetermined value of CVT ratio is comparable to the lowest gear ratio in the conventional discrete type transmission.
b) Due to difficulty in establishing, with good precision, a desired value of CVT ratio, a usable range of CVT ratios is narrowed to leave a relatively wide margin at each of mechanical limits as illustrated in FIG. 27. Fuel economy may be improved by extending the usable range of CVT ratios sufficiently to provide a wide range of CVT ratios for fuel efficient operation with CVT ratios at high vehicle speeds. However, so extending the usable range of CVT ratios requires an increase in size of a CVT unit to provide the margins, pushing up manufacturing cost. If such increase in size of a CVT unit is not desired, one cannot accomplish an improvement in fuel economy as high as expected.
c) Difficulty in establishing, with good precision, a desired value of CVT ratio makes it difficult for an infinitely variable transmission (IVT) to have a geared neutral point (GNP). The IVT includes a CVT in combination with a constant ratio transmission and a planetary gearing mechanism. The GNP is a point at which an infinitely great ratio is established to provide zero driving force.
An object of the present invention is to provide a method and system for enhanced CVT control with intelligent noise management to accomplish a desired CVT ratio with excellent precision without hunting and standing deviation.